Operating System Security
John Mitchell
Operating System Functions
OS is a resource allocator• Manages resources, decides between
conflicting requests
OS is a control program• Controls execution of programs to prevent
errors and improper use of the system
Security issues
Isolation• Separate processes execute in separate memory space• Process can only manipulate allocated pages
Access control• When can process create or access a file? • Create or read/write to socket? • Make a specific system call?
Protection problem • Ensure that each object is accessed correctly and only
by those processes that are allowed to do so Comparison between different operating systems
• Compare protection models: which model supports least privilege most effectively?
• Which system best enforces its protection model?
Outline
Access Control Concepts• Matrix, ACL, Capabilities• Multi-level security (MLS)
OS Mechanisms• Multics
– Ring structure
• Amoeba– Distributed, capabilities
• Unix– File system, Setuid
• Windows– File system, Tokens, EFS
• SE Linux– Role-based, Domain type enforcement
Topics for next lecture• Secure OS
– Methods for resisting stronger attacks
• Cryptographic file systems
• Assurance– Orange Book, TCSEC– Common Criteria– Windows 2000
certification• Some Limitations
– Information flow– Covert channels
Access control
Context• System knows who the user is
– User has entered a name and password, or other info
• Access requests pass through gatekeeper– OS must be designed so monitor cannot be bypassed
? ResourceUser
process
Decide whether user can apply operation to resource
Referencemonitor
Access control matrix [Lampson]
File 1 File 2 File 3 … File n
User 1
read write - - read
User 2
write write write - -
User 3
- - - read read
…
User m
read write read write read
Subjects
Objects
Two implementation concepts
Access control list (ACL)• Store column of matrix with the resource
Capability• User holds a “ticket” for each resource• Two variations
– store row of matrix with user, under OS control– unforgeable ticket in user space
File 1 File 2 …
User 1 read write -
User 2 write write -
User 3 - - read
…
User m
read write write
Access control lists are widely used, often with groups
Some aspects of capability concept are used in Kerberos, …
Capabilities
Operating system concept• “… of the future and always will be …”
Examples• Dennis and van Horn, MIT PDP-1 Timesharing• Hydra, StarOS, Intel iAPX 432, Eros, …• Amoeba: distributed, unforgeable tickets
References• Henry Levy, Capability-based Computer
Systemshttp://www.cs.washington.edu/homes/levy/capabook/
• Tanenbaum, Amoeba papers
ACL vs Capabilities
Access control list• Associate list with each object• Check user/group against list• Relies on authentication: need to know user
Capabilities• Capability is unforgeable ticket
– Random bit sequence, or managed by OS– Can be passed from one process to another
• Reference monitor checks ticket– Does not need to know identify of user/process
ACL vs Capabilities
Process P
User U
Process Q
User U
Process R
User U
Process P
Capabilty c,d
Process Q
Process R
Capabilty c
Capabilty c
ACL vs Capabilities
Delegation• Cap: Process can pass capability at run time• ACL: Try to get owner to add permission to list
Revocation• ACL: Remove user or group from list• Cap: Try to get capability back from process?
– Possible in some systems if appropriate bookkeeping• OS knows what data is capability• If capability is used for multiple resources, have to
revoke all or none …• Other details …
Roles (also called Groups)
Role = set of users• Administrator, PowerUser, User, Guest• Assign permissions to roles; each user gets
permission
Role hierarchy• Partial order of roles• Each role gets
permissions of roles below• List only new permissions given to each role
Administrator
Guest
PowerUser
User
Groups for resources, rights
Permission = right, resource Permission hierarchies
• If user has right r, and r>s, then user has right s• If user has read access to directory, user has read
access to every file in directory
Big problem in access control• Complex mechanisms require complex input• Difficult to configure and maintain• Roles, other organizing ideas try to simplify
problem
Multi-Level Security (MLS) Concepts
Military security policy– Classification involves sensitivity levels,
compartments– Do not let classified information leak to
unclassified files
Group individuals and resources• Use some form of hierarchy to organize
policy
Other policy concepts• Separation of duty• “Chinese Wall” Policy
Military security policy
Sensitivity levels
Top Secret
Secret
Confidential
Restricted
Unclassified
Compartments
Satellite data
AfghanistanMiddle East
Israel
Military security policy
Classification of personnel and data• Class = rank, compartment
Dominance relation • D1 D2 iff rank1 rank2
and compartment1 compartment2
• Example: Restricted, Israel Secret, Middle East Applies to
• Subjects – users or processes• Objects – documents or resources
Commercial version
Internal
Proprietary
Public
Product specifications
In productionOEM
Discontinued
Bell-LaPadula Confidentiality Model
When is it OK to release information? Two Properties (with silly names)
• Simple security property– A subject S may read object O only if C(O) C(S)
• *-Property– A subject S with read access to O may write object
P only if C(O) C(P)
In words,• You may only read below your classification and only write above your classification
Picture: Confidentiality
S
Public
Proprietary
Read below, write above
S
Public
Proprietary
Read above, write below
Biba Integrity Model
Rules that preserve integrity of information Two Properties (with silly names)
• Simple integrity property– A subject S may write object O only if C(S) C(O) (Only trust S to modify O if S has higher rank …)
• *-Property– A subject S with read access to O may write object P only
if C(O) C(P) (Only move info from O to P if O is more trusted than P)
In words,• You may only write below your classification and only read above your classification
Picture: Integrity
S
Public
Proprietary
Read above, write below
S
Public
Proprietary
Read below, write above
Problem: Models appear contradictory
Bell-LaPadula Confidentiality• Read down, write up
Biba Integrity• Read up, write down
Want both confidentiality and integrity• Contradiction is partly an illusion• May use Bell-LaPadula for some classification
of personnel and data, Biba for another– Otherwise, only way to satisfy both models is only
allow read and write at same classification
In reality: Bell-LaPadula used more than Biba model, e.g., Common Criteria
Other policy concepts
Separation of duty• If amount is over $10,000, check is only valid if
signed by two authorized people• Two people must be different• Policy involves role membership and
Chinese Wall Policy• Lawyers L1, L2 in Firm F are experts in banking • If bank B1 sues bank B2,
– L1 and L2 can each work for either B1 or B2– No lawyer can work for opposite sides in any case
• Permission depends on use of other permissions
These policies cannot be represented using access matrix
Example OS Mechanisms
Multics Amoeba Unix Windows SE Linux (briefly)
Multics
Operating System• Designed 1964-1967
– MIT Project MAC, Bell Labs, GE
• At peak, ~100 Multics sites• Last system, Canadian Department of
Defense, Nova Scotia, shut down October, 2000
Extensive Security Mechanisms• Influenced many subsequent systems
http://www.multicians.org/security.html
E.I. Organick, The Multics System: An Examination of Its Structure, MIT Press, 1972
Multics time period
Timesharing was new concept• Serve Boston area with one 386-based PC
F.J. Corbato
Multics Innovations
Segmented, Virtual memory• Hardware translates virtual address to real address
High-level language implementation• Written in PL/1, only small part in assembly lang
Shared memory multiprocessor• Multiple CPUs share same physical memory
Relational database• Multics Relational Data Store (MRDS) in 1978
Security• Designed to be secure from the beginning• First B2 security rating (1980s), only one for years
Multics Access Model
Ring structure• A ring is a domain in which a process executes• Numbered 0, 1, 2, … ; Kernel is ring 0• Graduated privileges
– Processes at ring i have privileges of every ring j > i
Segments• Each data area or procedure is called a segment• Segment protection b1, b2, b3 with b1 b2 b3
– Process/data can be accessed from rings b1 … b2– A process from rings b2 … b3 can only call segment at
restricted entry points
Multics process
Multiple segments• Segments are dynamically linked• Linking process uses file system to find segment• A segment may be shared by several processes
Multiple rings• Procedure, data segments each in specific ring• Access depends on two mechanisms
– Per-Segment Access Control• File author specifies the users that have access to it
– Concentric Rings of Protection• Call or read/write segments in outer rings• To access inner ring, go through a “gatekeeper”
Interprocess communication through “channels”
Amoeba
Distributed system• Multiple processors, connected by network• Process on A can start a new process on B• Location of processes designed to be transparent
Capability-based system• Each object resides on server• Invoke operation through message to server
– Send message with capability and parameters– Sever uses object # to indentify object– Sever checks rights field to see if operation is allowed– Check field prevents processes from forging capabilities
Server port Check fieldObj # Rights
Capabilities
Owner capability• When server creates object, returns owner cap.
– All rights bits are set to 1 (= allow operation)– Check field contains 48-bit rand number stored by server
Derived capability• Owner can set some rights bits to 0• Calculate new check field
– XOR rights field with random number from check field– Apply one-way function to calculate new check field
• Server can verify rights and check field– Without owner capability, cannot forge derived capability
Server port Check fieldObj # Rights
Protection by user-process at server; no special OS support needed
Unix file security
Each file has owner and group Permissions set by owner
• Read, write, execute• Owner, group, other• Represented by vector of four octal values
Only owner, root can change permissions• This privilege cannot be delegated or shared
Setid bits – Discuss in a few slides
rwx rwxrwx-
ownr grp othr
setid
Question
Owner can have fewer privileges than other• What happens?
– Owner gets access?– Owner does not?
Prioritized resolution of differencesif user = owner then owner permission else if user in group then group permission else other permission
Effective user id (EUID)
Each process has three Ids (+ more under Linux)• Real user ID (RUID)
– same as the user ID of parent (unless changed)– used to determine which user started the process
• Effective user ID (EUID)– from set user ID bit on the file being executed, or sys call– determines the permissions for process
• file access and port binding
• Saved user ID (SUID)– So previous EUID can be restored
Real group ID, effective group ID, used similarly
Process Operations and IDs
Root• ID=0 for superuser root; can access any file
Fork and Exec• Inherit three IDs, except exec of file with setuid bit
Setuid system calls • seteuid(newid) can set EUID to
– Real ID or saved ID, regardless of current EUID– Any ID, if EUID=0
Details are actually more complicated• Several different calls: setuid, seteuid, setreuid
Setid bits on executable Unix file
Three setid bits• Setuid – set EUID of process to ID of file
owner• Setgid – set EGID of process to GID of file• Sticky
– Off: if user has write permission on directory, can rename or remove files, even if not owner
– On: only file owner, directory owner, and root can rename or remove file in the directory
Example
…;…;exec( );
RUID 25 SetUID
program
…;…;i=getruid()setuid(i);…;…;
RUID 25EUID 18
RUID 25EUID 25
-rw-r--r--
file
-rw-r--r--
file
Owner 18
Owner 25
read/write
read/write
Owner 18
Compare to stack inspection
Careful with Setuid !• Can do anything
that owner of file is allowed to do
• Be sure not to– Take action for
untrusted user– Return secret data
to untrusted user
A 1
B 1
C 1
Note: anything possible if root; no middle ground between user and root
Setuid programming
We talked about this before … Be Careful!
• Root can do anything; don’ t get tricked• Principle of least privilege – change EUID
when root privileges no longer needed
Setuid scripts• This is a bad idea• Historically, race conditions
– Begin executing setuid program; change contents of program before it loads and is executed
Unix summary
Many of you may be used to this …• So probably seems pretty good• We overlook ways it might be better
Good things• Some protection from most users• Flexible enough to make things possible
Main bad thing• Too tempting to use root privileges• No way to assume some root privileges
without all root privileges
Access control in Windows (NTFS)
Some basic functionality similar to Unix• Specify access for groups and users
– Read, modify, change owner, delete
Some additional concepts• Tokens• Security attributes
Generally• More flexibility than Unix
– Can define new permissions– Can give some but not all administrator privileges
Sample permission options
Security ID (SID)• Identity (replaces
UID)– SID revision number– 48-bit authority value– variable number of
Relative Identifiers (RIDs), for uniqueness
• Users, groups, computers, domains, domain members all have SIDs
Permission Inheritance
Static permission inheritance (Win NT)• Initially, subfolders inherit permissions of folder• Folder, subfolder changed independently• Replace Permissions on Subdirectories command
– Eliminates any differences in permissions
Dynamic permission inheritance (Win 2000)• Child inherits parent permission, remains linked• Parent changes are inherited, except explicit settings• Inherited and explicitly-set permissions may conflict
– Resolution rules• Positive permissions are additive• Negative permission (deny access) takes priority
Tokens
Security Reference Monitor • uses tokens to identify the security context
of a process or thread
Security context• privileges, accounts, and groups associated
with the process or thread
Impersonation token • thread uses temporarily to adopt a different
security context, usually of another user
Security Descriptor
Information associated with an object• who can perform what actions on the object
Several fields• Header
– Descriptor revision number – Control flags, attributes of the descriptor
• E.g., memory layout of the descriptor
• SID of the object's owner• SID of the primary group of the object • Two attached optional lists:
– Discretionary Access Control List (DACL) – users, groups, …– System Access Control List (SACL) – system logs, ..
Example access request
User: MarkGroup1: Administrators
Group2: Writers
Control flags
Group SIDDACL PointerSACL Pointer Deny Writers Read, Write Allow Mark Read, Write
Owner SID
Revision Number
Access
token
Security descripto
r
Access request: write
Action: denied• User Mark requests write permission
• Descriptor denies permission to group
• Reference Monitor denies request
Impersonation Tokens (setuid?)
Process uses security attributes of another• Client passes impersonation token to server
Client specifies impersonation level of server• Anonymous
– Token has no information about the client
• Identification– server obtain the SIDs of client and client's privileges, but
server cannot impersonate the client
• Impersonation– server identify and impersonate the client
• Delegation– lets server impersonate client on local, remote systems
SELinux Security Policy Abstractions
Type enforcement• Each process has an associated domain• Each object has an associated type• Configuration files specify
– How domains are allowed to access types – Allowable interactions and transitions between domains
Role-based access control• Each process has an associated role
– Separate system and user processes
• configuration files specify – Set of domains that may be entered by each role
Outline
Access Control Concepts• Matrix, ACL, Capabilities• Multi-level security (MLS)
OS Mechanisms• Multics
– Ring structure
• Amoeba– Distributed, capabilities
• Unix– File system, Setuid
• Windows– File system, Tokens, EFS
• SE Linux– Role-based, Domain type enforcement
Topics for next lecture• Secure OS
– Methods for resisting stronger attacks
• Cryptographic file systems
• Assurance– Orange Book, TCSEC– Common Criteria– Windows 2000
certification• Some Limitations
– Information flow– Covert channels